1. Field of the Invention
The present invention relates to a process for the metathesis of alkenyl esters utilizing a homogeneous catalyst comprised of tungsten hexachloride and a tetraalkyltin.
2. Description of the Prior Art
Metathesis reactions using functionalized olefins have been the subject of considerable research since the discovery by van Dam et al. that methyl oleate can be metathesized into 9-octadecene and dimethyl 9-octadecenedioate (see J. Chem. Soc. Chem. Comm., 1221 (1972)). Using the homogeneous catalyst WCl.sub.6 /Sn(Me).sub.4, van Dam et al. reported a conversion of about 50 percent in two hours at 100.degree. C. and reactant:catalyst ratio of 75:1. Similar results were reported by van Dam et al. with methyl elaidate, methyl erucate, methyl undecenate, methyl linoleate and methyl linolenate. Nakamura, in Petrotech, 4, 623 (1981), also reported the metathesis and co-metathesis of methyl esters of unsaturated acids using catalysts based on tungsten hexachloride and other transition metals.
In addition to the metathesis of unsaturated esters wherein the unsaturation is present in the acyl moiety of the molecule, metathesis of alkenyl esters of monocarboxylic acids, i.e., esters wherein the unsaturation is located in the alcohol-derived moiety, using tungsten catalysts is also known. For example, Tsuji et al. (J. Organomet. Chem., 218, 69-80 (1981)) have metathesized oleyl acetate to obtain 9-octadecene and 1,18-diacetoxy-9-octadecene using WCl.sub.6 or WOCl.sub.4 as the primary catalyst with SnMe.sub.4, Cp.sub.2 TiMe.sub.2 or Cp.sub.2 TiClMe as co-catalyst. Similarly, 4-acetoxy-1-butane, 5-acetoxy-1-pentene and 6-acetoxy-1-hexene were cross metathesized with 2-hexene and cyclooctene using WCl.sub.6 /Sn(Me).sub.4 catalyst by Otton et al., (J. Mol. Catal., 8, 313-324 (1980)). J. C. Mol (Chemtech, April 1983, 250-255) metathesized esters with a double bond in the alcohol fragment (alkenyl esters) to produce ethylene and the corresponding .alpha.,.OMEGA.-diacetoxyalkene. Specifically, Mol reacted CH.sub.2 .dbd.CH(CH.sub.2).sub.n OOCCH.sub.3, where n=2, 3 or 8, at 70.degree. C. using a WCl.sub.6 /Sn(Me).sub.4 catalyst at a molar ratio (ester:WCl.sub.6) of 10:1. Conversions of 41-45 percent with selectivities of 88-95 percent to the desired CH.sub.3 COO(CH.sub.2).sub.n HC.dbd.CH(CH.sub.2).sub.n OOCCH.sub.3 were obtained.
With all of the above reactions involving alkenyl esters, very high catalyst levels are required. Molar ratios (ester:WCl.sub.6) of about 10:1 (10 mole percent) are typically required and, in some instances, 20 mole percent catalyst (ester:WCl.sub.6 ratio of 5:1) is necessary to achieve acceptable rates of reaction and conversion. Also, all of the prior art metathesis reactions involving alkenyl esters have utilized acetate esters.
It would be highly desirable if a process were available whereby alkenyl esters could be metathesized utilizing significantly lower levels of the costly tungsten hexachloride catalyst. It would be even more advantageous if acceptable reaction rates with high conversion of the alkenyl ester and high selectivity to the desired metathesis products were obtained.